1. Field of the Invention
The present invention relates to: a resin-coated carrier for an electrophotographic developer constituting a two-component developer used for developing an electrostatic latent image in electrophotography, electrostatic recording, electrostatic printing, or the like; and a two-component developer and a replenishing developer each of which contains the resin-coated carrier as a constituent.
2. Related Background Art
In recent years, biodegradable polymer materials have been finding a wide variety of applications including medical materials, drug delivery systems, and environmentally compatible materials. In recent years, in addition to those applications, the biodegradable polymer materials have been requested to provide new functions, and hence various studies have been made. In particular, the introduction of a chemically modifiable functional group into a molecule of a polyhydroxyalkanoate typified by polylactic acid has been examined. For example, there has been reported a compound into which a carboxyl group or a vinyl group is introduced.
For example, polymalic acid has been known as a polyhydroxyalkanoate having a carboxyl group at a side chain thereof. An α-type represented by the chemical formula (14) and a β-type represented by the chemical formula (15) have been known as polymers of polymalic acid depending on the form of a polymer.

Of those, a polymer obtained by ring-opening polymerization of a benzyl ester of β-malolactone represented by the following chemical formula (16) is disclosed in U.S. Pat. No. 4,265,247 as β-type polymalic acid or a copolymer thereof.
(In the formula, R16 represents a benzyl group.)
In addition, a polymer obtained by copolymerization of a six-membered ring diester monomer and a glicolide or lactide as a cyclic diester or a lactone as an intramolecular ring closure reaction ester of ω-hydroxycarboxylic acid represented by the chemical formula (17) is disclosed in Japanese Patent Application Laid-Open No. H02-3415 as a copolymer containing any one of other hydroxyalkanoic acids typified by α-type polymalic acid-glycolic acid copolymer and glycolic acid.
(In the formula, R17 represents a lower alkyl group such as a methyl group, an ethyl group, an n-propyl group, an isopropyl group, or a t-butyl group, or a benzyl group.)
“Macromolecules” 2000, vol. 33, No. 13, p. 4619 discloses that 7-oxo-4-oxepancarboxylate is subjected to ring-opening polymerization to produce a polymer having an ester group at a side chain thereof, and the polymer is further subjected to hydrogenolysis to produce a polymer having a carboxylic acid at a side chain thereof as a polyhydroxyalkanoate having a carboxyl group at a side chain thereof.
“Biomacromolecules” 2000, vol. 1, p. 275 discloses a polymer in which a benzyloxycarbonyl group is introduced into a methylene group at position a of a carbonyl group in the main chain of poly(ε-caprolactone), the polymer being obtained by: allowing lithium diisopropylamide to react with poly(ε-caprolactone); and allowing the resultant to react with benzyl chloroformate. “Macromolecular Bioscience” 2004, vol. 4, p. 232 discloses a polymer in which a (benzyloxycarbonyl)methyl group is introduced into a methylene group at position a of a carbonyl group in the main chain of polylactic acid, the polymer being obtained by: allowing lithium diisopropylamide to react with polylactic acid; and allowing the resultant to react with benzyl bromoacetate.
“Polymeric Materials Science & Engineering” 2002, vol. 87, p. 254 discloses, as a polyhydroxyalkanoate having a vinyl group at a side chain thereof, a polymer obtained by ring-opening polymerization of α-allyl(δ-valerolactone).
Similarly, “Polymer Preprints” 2002, vol. 43, No. 2, p. 727 discloses, as a polyhydroxyalkanoate having a vinyl group at a side chain thereof, a polymer obtained by ring-opening polymerization of 3,6-diallyl-1,4-dioxane-2,5-dione as a six-membered ring diester monomer.
There has been reported a polymer having a new function into which a structure providing functional properties for a polyhydroxyalkanoate into which a chemically modifiable functional group is introduced as described above is introduced. “International Journal of Biological Macromolecules” 1999, vol. 25, p. 265 discloses the following. A copolymer of α-type malic acid and glycolic acid is obtained by ring-opening polymerization of a cyclic dimer of α-type malic acid and glycolic acid, and the resultant polymer is deprotected to obtain a polyester having a carboxyl group at a side chain thereof. Tripeptide is chemically modified to the carboxyl group at the side chain, and the resultant polymer is evaluated for cell adhesion. At this time, a good result is obtained.
With regard to a method of obtaining a polyhydroxyalkanoate represented by the chemical formula (21) described below involving oxidation cleavage of a carbon-carbon double bond of a side chain of a polyhydroxyalkanoate represented by the chemical formula (6) described below as a staring material with an oxidizing agent, for example, J. Chem. Soc., Perkin. Trans., 1973, vol. 1, p. 806 discloses a method involving the use of a permanganate, Org. Synth., 1963, vol. 4, p. 698 discloses a method involving the use of a dichromate, J. Org. Chem., 1981, vol. 46, p. 19 discloses a method involving the use of a periodate, Japanese Patent Application Laid-Open No. S59-190945 discloses a method involving the use of nitric acid, and J. Am. Chem. Soc., 1959, vol. 81, p. 4273 discloses a method involving the use of ozone. In addition, “Macromolecular chemistry” 2001, vol. 4, p. 289-293 has reported a method of obtaining a carboxylic acid involving subjecting a carbon-carbon double bond of a side chain terminal of a polyhydroxyalkanoate produced by using a microorganism to a reaction under an acid condition by means of potassium permanganate as an oxidizing agent.
Meanwhile, electrophotography has been conventionally known, which involves: forming an electrical latent image on the surface of a photoconductive material by means of electrostatic means; and developing the latent image to form an image. A large number of methods based on the electrophotography have been known. That is, the electrophotography generally involves: forming an electrical latent image on a photosensitive member by using a photoconductive substance and various means; allowing a finely pulverized voltage detection material called toner carried and conveyed by a developer carrier to adhere to the latent image to form a toner image corresponding to an electrostatic latent image; transferring the toner image onto the surface of an image support such as paper as required; and fixing the toner image by means of heat, pressure, or solvent vapor to provide a copied product.
Known examples of a method of visualizing an electrical latent image by means of toner include a powder cloud method, a cascade development method, a magnetic brush method, and a method involving the use of conductive magnetic toner. In addition to the above methods, a so-called J/B development method has been known, which involves applying a bias electric field composed of an AC component and a DC component to a space between a developer carrier (developing sleeve) and a photoconductive layer to perform development. A representative method for the development method is the magnetic brush method. The magnetic brush method involves the use of a two-component developer composed of toner and a magnetic carrier. When particles having magnetic properties made of steel, ferrite, and the like are used as a carrier, the developer containing the magnetic carrier is held on a developer carrier having a magnet in it. Thus, the developer is arranged in a brush fashion on the developer carrier by a magnetic field of the magnet. Then, when the magnetic brush contacts the surface of an electrostatic latent image on a photoconductive layer, only the toner in the developer is attracted from the brush to the electrostatic latent image, whereby the electrostatic latent image is developed.
Carriers constituting a two-component developer applicable to the above development method are roughly classified into a conductive carrier and an insulating carrier. Oxidized or unoxidized iron powder is generally used as the conductive carrier. However, in a developer containing the iron powder carrier as a constituent, triboelectric chargeability to toner is unstable. On the other hand, a resin-coated carrier obtained by evenly coating the surface of a carrier core material made of a ferromagnetic substance such as iron, nickel, or ferrite with an insulating resin is a representative example of the insulating carrier. A developer using such an insulating resin-coated carrier has a significantly reduced frequency of fusion of a toner particle to a carrier surface as compared to the conductive carrier described above. In addition, the developer facilitates the control of triboelectric chargeability between toner and the carrier, is excellent in durability, and has a long lifetime, so it is particularly suitable for a high-speed electronic copying machine.
Various properties are requested for an insulating carrier. Of those, examples of particularly important properties include appropriate chargeability, impact resistance, abrasion resistance, good adherence between a core material and a coating material, and uniformity of a charge distribution. In order to prevent a spent carrier such as toner adhesion, a proposal has been made, in which a resin having small surface energy is used as a coating layer material to increase the durability of a developer. That is, it is said that a carrier coated with a silicone resin, a fluorine resin, or the like is hardly spent and has a long lifetime as a developer. As described in Japanese Patent Application Laid-Open No. S62-121462, a resin coating layer has been improved by adding various silane coupling agents to a condensation reaction type silicone resin.
A carrier for electrophotography is also problematic in terms of durability and environmental stability. With regard to the durability, the rise of an initial charge amount is generally slow, so fogging tends to occur in an initial image or an image density tends to be high. In addition, in long-term copy duration, a charge amount reduces, so fogging tends to occur in an image or an image density tends to be high. With regard to the environmental stability, a charge amount tends to reduce in a high-humidity environment, so fogging tends to occur in an image, an image density tends to be high, or toner scattering tends to occur. In a low-humidity environment, a charge amount tends to increase, so an image density tends to reduce.